Method, apparatus, and system providing an imager with pixels having extended dynamic range

ABSTRACT

The dynamic range of a pixel is increased by using selective photosensor resets during a frame time of image capture at a timing depending on the light intensity that the pixel will be exposed to during the frame time. Pixels that will be exposed to high light intensity are reset later in the frame than pixels that will be exposed to lower light intensity.

FIELD OF THE INVENTION

The various embodiments relate generally to imagers, and moreparticularly to an imager that includes a pixel with extended dynamicrange.

BACKGROUND

Imagers, such as complementary metal oxide semiconductor (CMOS) imagers,are commonly used in photo-imaging applications. A typical imagerincludes a focal plane array of pixels. Each of the cells includes aphotoconversion device or photosensor such as, for example, a photogate,photoconductor, or photodiode, for generating and accumulatingphoto-generated charge in a portion of the substrate of the array. Areadout circuit is connected to each pixel and includes at least anoutput transistor, which receives photo-generated charges from a dopeddiffusion region and produces an output signal that is read-out througha pixel access transistor.

CMOS imaging circuits, processing steps thereof, and detaileddescriptions of the functions of various CMOS elements of an imagingcircuit are described, for example, in U.S. Pat. Nos. 6,140,630,6,376,868, 6,310,366, 6,326,652, 6,204,524, and U.S. Pat. No. 6,333,205,all of which are assigned to Micron Technology, Inc. The disclosures ofeach of the forgoing are hereby incorporated by reference herein intheir entirety.

FIG. 1 illustrates a typical four transistor pixel 50 utilized in animager, such as a CMOS imager. The pixel 50 includes a photosensor 52(e.g., photodiode, photogate, etc.), a storage node configured as afloating diffusion region N, transfer transistor 54, reset transistor56, source follower transistor 58 and row select transistor 60. Thephotosensor 52 is connected to the floating diffusion region N by thetransfer transistor 54 when the transfer transistor 54 is activated by atransfer control signal TX. The reset transistor 56 is connected betweenthe floating diffusion region N and an array pixel supply voltage VAA. Areset control signal RESET is used to activate the reset transistor 56,which resets the floating diffusion region N to a known state as isknown in the art.

The source follower transistor 58 has its gate connected to the floatingdiffusion region N and is connected between the array pixel supplyvoltage VAA and the row select transistor 60. The source followertransistor 58 converts the charge stored at the floating diffusionregion N into an electrical output voltage signal. The row selecttransistor 60 is controllable by a row select signal ROW for selectivelyoutputting the output voltage signal OUT from the source followertransistor 58. For each pixel 50, two output signals are conventionallygenerated, one being a reset signal V_(rst) generated after the floatingdiffusion region N is read, the other being an image signal V_(sig)generated after charges are transferred from the photosensor 52 to thefloating diffusion region N.

FIG. 2 shows an imager 200 that includes an array 230 of pixels (such asthe pixel 50 illustrated in FIG. 1) and a timing and control circuit232. The timing and control circuit 232 provides timing and controlsignals for enabling the reading out of signals from pixels of the array230 in a manner commonly known to those skilled in the art. The array230 has dimensions of M rows by N columns of pixels, with the size ofthe array 230 depending on a particular application.

Signals from the imager 200 are typically read out a row at a time usinga column parallel readout architecture. The timing and control circuit232 selects a particular row of pixels in the array 230 by controllingthe operation of a row addressing circuit 234 and row drivers 240.Signals stored in the selected row of pixels are provided to a readoutcircuit 242 in the manner described above. The signal read from each ofthe columns is then read out sequentially using a column addressingcircuit 244. Differential pixel signals (Vrst, Vsig) corresponding tothe pixel reset signal and image pixel signal are provided as respectiveoutputs V_(out1), V_(out2) of the readout circuit 242.

The pixels 50, of pixel array 230, have a characteristic dynamic range.Dynamic range refers to the range of incident light that can beaccommodated by a pixel in a single image frame. It is desirable to havepixels with a high dynamic range to image scenes that generate highdynamic range incident signals, such as indoor rooms with windows to theoutside, outdoor scenes with mixed shadows and bright sunshine, andnight-time scenes combining artificial lighting and shadows.

The dynamic range for a pixel is commonly defined as the ratio of itslargest non-saturating signal to the standard deviation of its noiseunder dark conditions. The dynamic range is limited on an upper end bythe charge saturation level of the pixel photosensor, and on a lower endby noise imposed limitations and/or quantization limits of theanalog-to-digital converter used to produce a digital signal from analogpixel signals. When the dynamic range of a pixel is too small toaccommodate the variations in light intensities of the imaged scene,e.g. by having a low saturation level, image distortion occurs.

One approach to increasing dynamic range is to provide structures toincrease dynamic range, which includes structures for signal companding,multiple signal storage, and signal controlled reset. Compandinginvolves compressing and subsequently expanding a signal to increase thedynamic range, but suffers from drawbacks such as requiring a non-linearoutput that hampers subsequent processing and causes increased pixelfixed pattern noise (FPN), a dip in the signal to noise ratio (SNR) atthe knee point, and low contrast at high brightness. Structuresproviding multiple signal storage and signal controlled reset may not bepractical because they require an increase in die area due to additionalcolumn circuitry.

Another approach to increase dynamic range is to use multiple imagecaptures with different integration times. Dual capture, for example, isrelatively simple to implement, but suffers from an SNR dip at the kneepoint of the collected charge relative to output signal. A multiplecapture approach that requires more than two image captures is quitedifficult to implement and requires high speed non-destructive readoutalong with on-chip memory and additional column circuitry.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 shows a portion of a conventional imager;

FIG. 2 is a block diagram of a conventional imager;

FIG. 3 shows pixels divided into brightness zones according to anembodiment;

FIG. 4 shows a zone bounded by various coordinates according to anembodiment;

FIG. 5 shows zones having complex shapes that may be approximated byshapes according to an embodiment;

FIG. 6 shows a portion of an imager constructed in accordance with anembodiment;

FIG. 7 is a timing diagram for operations of the imager shown in FIG. 6;

FIG. 8 is a graph of signal to noise ratio versus photocurrent for dualsampling and quadruple sampling operations according to embodiments;

FIG. 9 shows a portion of an imager constructed in accordance withanother embodiment;

FIG. 10 is a timing diagram for operations of the imager shown in FIG.9;

FIG. 11 shows a global shutter sensor window;

FIG. 12 shows selected brightness zones and generated false brightnesszones according to an embodiment;

FIG. 13 shows a portion of a shared pixel imager constructed inaccordance with an embodiment;

FIG. 14 is a timing diagram for operations of the imager shown in FIG.13; and

FIG. 15 illustrates a system suitable for use with any one of theembodiments.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings, which are a part of the specification, and inwhich is shown by way of illustration various embodiments whereby theinvention may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to make and use theinvention. It is to be understood that other embodiments may beutilized, and that structural, logical, and electrical changes, as wellas changes in the materials used, may be made without departing from thespirit and scope of the present invention.

The term “pixel” refers to a picture element unit cell containing aphotosensor and transistors for converting light radiation to anelectrical signal. For purposes of illustration, a representative pixelis illustrated in the figures and description herein and, typically,fabrication of all pixels in an imager will proceed simultaneously in asimilar fashion.

Although various embodiments are described herein with reference to thearchitecture of one pixel, it should be understood that this isrepresentative of a plurality of pixels in an array of an imager. Thefollowing detailed description is, therefore, not to be taken in alimiting sense, and the scope of the invention is defined only by theappended claims.

The disclosed embodiments increase the dynamic range of an imager usingmultiple pixel resets to reset the accumulated charge on the pixelphotosensor. The accumulated charge in the photosensor is reset atvarying times during a frame time depending on the light intensity thatthe pixel will be exposed to during the frame time. Pixels that will beexposed to high light intensity are reset later in the frame than pixelsthat will be exposed to lower light intensity. This reduces the value ofthe pixel image signal produced by pixels exposed to high lightintensity relative to pixels exposed to low light intensity andincreases the dynamic range as the pixel image signal exposed to highlight intensity can be expanded during digital signal processing.

The light intensity that the pixels will be exposed to during the frametime is initially determined. This may be accomplished using the samelatency approach as is used in automatic exposure techniques, which areknown in the art. The light intensity that each pixel in the array willbe exposed to may be mapped together with the automatic exposuretechnique using the image processor or a camera processor.

Preset thresholds of light intensity are used to designate a pluralityof light intensity ranges. Each light intensity range is associated witha brightness value. The light intensity may be divided into two, three,four, or more ranges. Each pixel exposed to light intensity falling intoa range is assigned the brightness value associated with that range andthat value is associated with a time of application of a photosensorreset signal for the pixel.

In one embodiment, the brightness value for each pixel is stored tomemory. In another embodiment, to save memory, pixels having the samebrightness value may be grouped into zones of pixels and the dimensionsof the zones may be stored to memory. FIG. 3, for example, shows animage field, divided into four brightness zones, 1, 2, 3, and 4 in whichthe corresponding pixels (not shown) may be grouped. Each of the fourzones, 1, 2, 3, and 4, may therefore be made up of pixels assigned withthe same brightness value.

FIG. 4, for example, shows one zone 5 of same brightness value pixels inwhich the n^(th) row of pixels is bounded on the left by the coordinateX_(n1) and on the right by the coordinate X_(n2). The zone is bounded onthe top by row “p” and at the bottom by row “j”. Therefore, rather thanstoring the same brightness value for each pixel in the zone, twohorizontal coordinates for each row and two vertical coordinates for theentire brightness zone can be saved to memory, thus reducing the size ofthe required memory.

In another embodiment, to save memory, brightness zones with the samebrightness value but having complex shapes may be approximated by one ormore shapes having easily defined areas, such as circles, rectangles,ovals, triangles, annular rings, and others. FIG. 5, for example, showsa complex shape 10, that may be approximated by a circle 12. The area ofthe circle 12 may be saved to memory by saving the “x” and “y”coordinates of the center of the circle and the radius R of the circle.FIG. 5 also shows another complex shape 14 that may be approximated by arectangle 16. The area of the rectangle may be saved to memory by savingthe “x” and “y” coordinates of one corner of the rectangle and thelength dy and width dx of the rectangle.

Once a brightness value has been assigned to each pixel and the valueshave been stored to memory either directly or as zone coordinates, thearray of pixels may be operated to capture an image. FIG. 6 shows apixel 350 for use in a rolling shutter imager constructed in accordancewith an embodiment of the invention. The pixel 350 includes aphotosensor 352 (e.g., photodiode, photogate, etc.), floating diffusionregion N for storing charge, transfer transistor 354, reset transistor356, source follower transistor 358, row select transistor 360, andphotosensor reset transistor 364. The drain of the transfer transistor354 is connected to the photosensor 352. The source of the transfertransistor 354 is connected to the floating diffusion region N. Thesource of the photosensor reset transistor 364 is connected to the gateof the transfer transistor 354. The source of the reset transistor 356is connected to the floating diffusion region N. The drain of the resettransistor 356 is connected to the supply voltage VAA. The gate of thesource follower transistor 358 is connected to the floating diffusionregion N. The drain of the source follower transistor 358 is connectedto the supply voltage VAA. The source of the source follower transistor358 is connected to the drain of the row select transistor 360. Anenable signal EN is applied to the drain of the photosensor resettransistor 364. A transfer control signal TX is applied to the gate ofthe photosensor reset transistor 364. A reset control signal RESET isapplied to the gate of the reset transistor 356. A row select signal ROWis applied to the gate of the row select transistor 360.

FIG. 7 is an example of a timing diagram which may be used to operatethe rolling shutter sensor pixel 350 of FIG. 6. The operation of thepixel 350 is now described with reference to FIGS. 6 and 7.

The reset control signal RESET is pulsed once during each frame time TFfor each brightness value that may be assigned to each pixel. In thisembodiment, four brightness values are used, and therefore, four resetcontrol signals RESET₁, RESET₂, RESET₃, RESET₄ will be pulsed during theframe time T_(F).

The transfer control signal TX is pulsed and applied to the photosensorreset transistor 364 at the same time as the reset control signalsRESET₂, RESET₃, RESET₄ to turn on transfer gate 354 when an enablesignal EN is present at the photosensor reset transistor 364, and thusreach the photosensor while the reset transistor 356 is on. The transfercontrol signal TX activates the photosensor reset transistor 364.

Depending on the brightness value assigned to the pixel 350, a dedicatedcolumn controller pulses an enable signal EN[2], EN[3], or EN[4] duringthe pulsing of one of the reset control signals RESET₂, RESET₃, RESET₄and transfer control signals TX. The enable signal EN[2], EN[3], orEN[4] activates the transfer transistor 354 to transfer any accumulatedcharge from the photosensor 352 to the floating diffusion region N,which is then under reset, and thus resets the photosensor 352 torestart charge integration. The enable signal EN[1], EN[2], EN[3], orEN[4] pulse should be shorter than the transfer control signal TX pulseto ensure that the transfer gate transistor 354 is completely off beforethe reset control signal RESET resets the floating diffusion region N,which discards the charge transferred from the photosensor 352. Thisensures that the transfer transistor 354 is completely off during chargeintegration.

If the brightness value assigned to the pixel 350 is low, the pixel 350will receive only the first enable signal EN[1] to reset and read outthe charge on the photosensor 352 at the end of the frame time T_(F) sothat the photosensor 352 may accumulate charge over a first integrationtime T₁, which is equal to the entire frame time T_(F). Regardless ofthe brightness value assigned, each pixel at least receives the firstenable signal EN[1] during each frame time T_(F) to reset and read outthe charge on the photosensor 352. If the brightness value assigned tothe pixel 350 is of a lower intermediate value, the pixel 350 willreceive the second enable signal EN[2] to reset the photosensor near themiddle of the frame time T_(F) so that the photosensor 352 willaccumulate charge over a shorter integration time T₂. If the brightnessvalue assigned to the pixel 350 is of a higher intermediate value, thepixel 350 will receive the third enable signal EN[3] to reset thephotosensor towards the end of the frame time T_(F) so that thephotosensor 352 will accumulate charge over an even shorter integrationtime T₃. If the brightness value assigned to the pixel 350 is high, thepixel 350 will receive the fourth enable signal EN[4] to reset thephotosensor 352 near the end of the frame time T_(F), so that thephotosensor 352 will accumulate charge over the shortest integrationtime T₄.

Pixel reset occurs at the end of the frame time T_(F) for a given row.The reset control signal RESET is pulsed at RESET₁ to the gate of thereset transistor 356 to reset the floating diffusion region N. The pixelreset signal V_(rst) is sampled during RESET₁ by applying the charge onthe floating diffusion region to the gate of the source followertransistor 358 to generate the pixel reset signal V_(rst). A row selectsignal ROW is applied to the row select transistor 360 to output thepixel reset signal V_(rst). The pixel reset signal V_(rst) is thenstored by column readout circuitry (not shown) connected to a columnline, which is connected to the pixel 350.

After this, the transfer control signal TX and the enable signal EN[1]are pulsed to transfer any charge accumulated from the photosensor 352to the floating diffusion region N. The charge on the floating diffusionregion N is applied to the gate of the source follower transistor 358 togenerate the pixel image signal V_(sig). The row select signal ROW isapplied to the row select transistor 360 to output the pixel imagesignal V_(sig). The pixel image signal V_(sig) is stored in columncircuits (not shown) connected to the column line, which is connected topixel 350. The next integration period for pixels in a given row startswhen EN[1] is off.

The pixel reset signal V_(rst) and the pixel image signal V_(sig) arethen subtracted to produce a differential signal and the differentialsignal is digitized and sent to an image processor. The gain of thedigital signal is then adjusted by the image processor using a valuethat correlates to the brightness value to represent the actual lightintensity to which the pixel 350 was exposed.

The floating diffusion region N may therefore store similar amounts ofcharge and cause the source follower transistor 358 to produce pixelimage signals V_(sig) with similar values regardless of whether thepixel is exposed to low light, intermediate light, or bright light.Also, resetting the charge at varying times may prevent the floatingdiffusion region N from becoming saturated and may thus preventblooming.

Each of the four zones, 1, 2, 3, and 4, shown in FIG. 2 is made up ofpixels assigned with the same brightness value, and therefore, everypixel in a particular zone should be exposed to a light intensity thatfalls within the light intensity thresholds of the light intensity rangeassigned to that zone. The light intensity range should correspond tothe saturation level for a given integration time, where the integrationtime is fixed by the enable signal EN sent to the pixels. Using morezones, e.g. using more, smaller light intensity ranges and moreintegration times, results in less SNR dip. The SNR dip when theintegration time is switched from T₁ to T₂ is calculated by theequation:SNR _(dip)=10*log(T ₁ /T ₂)

FIG. 8 shows SNR versus photocurrent when using two zones for dualsampling 802 and four zones for quadruple sampling 804. It can be seenfrom FIG. 8 that dual sampling 802 causes SNR to drop 20 dB with 36 dBDR extension. The quadruple sampling 804 causes SNR to drop only 5 dBwith the same 36 dB DR extension. As is apparent from the foregoingdiscussion, although one embodiment has been described in which fourbrightness values and four associated photosensor reset signals EN[1],EN[2], EN[3], EN[4] are employed, a larger or smaller number ofbrightness levels and associated reset signals may be employed in any ofthe described embodiments.

FIG. 9 shows a global shutter pixel 550 for use in an imager constructedin accordance with another embodiment. The pixel 550 includes aphotosensor 552 (e.g., photodiode, photogate, etc.), a floatingdiffusion region N, an anti-blooming transistor 554, a reset transistor556, a source follower transistor 558, a row select transistor 560, atransfer transistor 562, and a photosensor reset transistor 564. Thephotosensor 552 is connected to the drain of the anti-bloomingtransistor 554 and the drain of the transfer transistor 562. The gate ofthe anti-blooming transistor 554 is connected to the source of thephotosensor reset transistor 564. The source of the anti-bloomingtransistor 554 is connected to the supply voltage VAA. The source of thephotosensor reset transistor 562 is connected to the floating diffusionregion N. The source of the reset transistor 556 is connected to thefloating diffusion region N. The drain of the reset transistor 556 isconnected to the supply voltage VAA. The gate of the source followertransistor 558 is connected to the floating diffusion region N. Thedrain of the source follower transistor 558 is connected to the supplyvoltage VAA. The source of the source follower transistor 558 isconnected to the drain of the row select transistor 560. An enablesignal EN is applied to the drain of the photosensor reset transistor564. A transfer control signal TX is applied to the gate of thephotosensor reset transistor 562. A floating diffusion reset controlsignal NRST is applied to the gate of the reset transistor 556. Aphotosensor reset control signal PRST is applied to the gate of thephotosensor reset transistor. A row select signal ROW is applied to thegate of the row select transistor 560.

FIG. 10 is a timing diagram illustrating the timing of the signals tooperate the pixel 550 of FIG. 9. The operation of the pixel 550 is nowdescribed with reference to FIGS. 9 and 10.

The pixel 550 is assigned a brightness value and the brightness value isstored to memory as described above. In the illustrated embodiment, thepixel 550 is assigned one of four brightness values.

A photosensor reset control signal PRST is pulsed four times (once foreach of four brightness values that may be assigned) and applied to thegate of the photosensor reset transistor 564.

A dedicated column controller pulses a first enable signal EN[1] duringthe first photosensor reset control signal PRST₁ pulse and, depending onthe brightness value assigned to the pixel 550, may pulse another enablesignal EN[2], EN[3], or EN[4] during the pulsing of one of the secondPRST₂, third PRST₃, or fourth PRST₄ photosensor reset control signalPRST pulses, respectively. The enable signal EN[1], EN[2], EN[3], orEN[4] and read signal PRST activate the anti-blooming transistor 554 toreset the charge on the photosensor 552. The enable signal EN[1], EN[2],EN[3], or EN[4] pulse should be shorter than the photosensor resetcontrol signal PRST pulse to ensure that the anti-blooming transistor554 is completely off before charge integration begins.

Similarly to the embodiment described above, the pixel 550 will receivethe first enable signal EN[1], and based on the assigned brightnessvalue, may receive a second enable signal EN[2], third enable signalEN[3], or fourth enable signal EN[4], to reset the photosensor 552 atthe beginning, near the middle, towards the end, or near the end,respectively, of the frame time TF.

At the end of the frame time T_(F), the transfer control signal TX ispulsed and applied to the transfer transistor 562 to transfer any chargeaccumulated during the integration period from the photosensor 552 tothe floating diffusion region N. The charge on the floating diffusionregion N is applied to the gate of the source follower transistor 558 togenerate the pixel image signal V_(sig). The row select signal ROW isapplied to the row select transistor 560 to output the pixel imagesignal V_(sig). The pixel image signal V_(sig) is stored in columncircuitry (not shown) connected to a column line, which is connected tothe pixel 350.

The floating diffusion region reset control signal NRST is pulsed andapplied to the gate of the reset transistor 556 to reset the floatingdiffusion region N at the row readout time, which happens once in theframe time T_(F). The charge on the floating diffusion region N isapplied to the gate of the source follower transistor 558 to generatethe pixel reset signal V_(rst). A row select signal ROW is applied tothe row select transistor 560 to output the pixel reset signal V_(rst).The pixel reset signal V_(rst) is stored in column circuitry (not shown)connected to a column line, which is connected to the pixel 550.

The pixel reset signal V_(rst) and the pixel image signal V_(sig) arethen subtracted to produce a differential signal and the differentialsignal is digitized and sent to an image processor. The gain of thedigital signal is then adjusted by the image processor using a valuethat correlates to the brightness value to represent the actual lightintensity to which the pixel 550 was exposed.

It is desired that the dimensions of the pixel zones be rectangular whena global shutter sensor pixel is used because a global shutter sensorwindow is rectangular. FIG. 11 shows a global shutter sensor window 20.Furthermore, two or more zones of the same brightness can not beselected simultaneously because this will generate false brightnesszones. FIG. 12 shows a situation where brightness zones A₁ and A₂ areselected at the same time and false brightness zones B₁ and B₂ aregenerated. The false brightness zones B₁ and B₂ are generated becausethe column lines are the same for the areas A₁ and A₂ where differentintegration times are desirable and for areas B₁ and B₂, respectively,in which different integration times are not desirable. This situationmay be corrected by using more complicated timing, by dropping the falsereadouts in later processing, or by adjusting the gains.

A rolling shutter sensor with shared pixel may also be constructed inaccordance with an embodiment. FIG. 13 shows an example of a sharedpixel 750 for use in an imager constructed in accordance with anembodiment of the invention. The pixel 750 includes a first photosensor752 (e.g., photodiode, photogate, etc.), a second photosensor 753, afloating diffusion region N, a reset transistor 756, a source followertransistor 758, a row select transistor 760, a first transfer transistor754, a second transfer transistor 755, a first photosensor resettransistor 764 and a second photosensor reset transistor 765. The drainof the first transfer transistor 754 is connected to the firstphotosensor 752. The drain of the second transfer transistor 755 isconnected to the second photosensor 753. The source of the firsttransfer transistor 754 is connected to the floating diffusion region N.The source of the second transfer transistor 755 is connected to thefloating diffusion region N. The source of the first photosensor resettransistor 764 is connected to the gate of the first transfer transistor754. The source of the second photosensor reset transistor 765 isconnected to the gate of the second transfer transistor 755. The sourceof the reset transistor 756 is connected to the floating diffusionregion N. The drain of the reset transistor 756 is connected to thesupply voltage VAA. The gate of the source follower transistor 758 isconnected to the floating diffusion region N. The drain of the sourcefollower transistor 758 is connected to the supply voltage VAA. Thesource of the source follower transistor 758 is connected to the drainof the row select transistor 760. A first enable signal EN1 is appliedto the drain of the first photosensor reset transistor 764. A secondenable signal EN2 is applied to the drain of the second photosensorreset transistor 765. A first transfer control signal TX1 is applied tothe gate of the first photosensor reset transistor 764. A secondtransfer control signal TX2 is applied to the gate of the secondphotosensor reset transistor 765. A reset control signal RESET isapplied to the gate of the reset transistor 756. A row select signal ROWis applied to the gate of the row select transistor 760. FIG. 14 is atiming diagram illustrating the timing of the signals to operate theshared rolling shutter sensor pixel 750 of FIG. 13. The operation of thepixel 750 is now described with reference to FIGS. 13 and 14.

The reset control signal RESET is pulsed once for each of the two frametimes T_(F1) and T_(F2) for each brightness value that may be assignedto each pixel. In this embodiment, four brightness values are used, andtherefore, eight reset control signals RESET₁, RESET₂, RESET₃, RESET₄,RESET5 ₅, RESET₆, RESET₇, RESET₈ will be pulsed during the frame timesT_(F1) and T_(F2).

The first transfer control signal TX1 is pulsed and applied to the firstphotosensor reset transistor 764 at the same time as the reset controlsignals RESET₃, RESET₅, RESET₇ to turn on the first transfer gate 754when an enable signal EN is present at the photosensor reset transistor764, and thus reach the first photosensor 752 while the reset transistor756 is on. The first transfer control signal TX1 activates the firstphotosensor reset transistor 764.

Similarly, the second transfer control signal TX2 is pulsed and appliedto the second photosensor reset transistor 765 at the same time as thereset control signals RESET₄, RESET₆, RESET₈ to turn on the secondtransfer gate 755 when an enable signal EN is present at the photosensorreset transistor 764, and thus reach the second photosensor 753 whilethe reset transistor 756 is on. The second transfer control signal TX2activates the second photosensor reset transistor 765.

A brightness value is assigned to the first photodiode 752 and thesecond photodiode 753 of the pixel 750. Depending on the brightnessvalues assigned to the first photodiode 752, a dedicated columncontroller pulses an enable signal EN[3], EN[5], or EN[7] during thepulsing of one of the reset control signals RESET₃, RESET₅, RESET₇ andfirst transfer control signal TX1. The enable signal EN[3], EN[5], orEN[7] activates the first transfer transistor 754 to transfer anyaccumulated charge from the first photosensor 752 to the floatingdiffusion region N, which is then under reset, and thus resets the firstphotosensor 752 to restart charge integration.

The same process is conducted for the second photodiode 753, which maybe assigned a brightness value different that the one assigned to thefirst photodiode 752. The dedicated column controller pulses an enablesignal EN[4], EN[6], or EN[8] during the pulsing of one of the resetcontrol signals RESET₄, RESET₆, RESET₈ and second transfer controlsignal TX2 to reset the second photosensor 753 to restart chargeintegration.

Similarly to the embodiments described above, the first photosensorreset transistor 764 will receive the first enable signal EN[1], andbased on the assigned brightness value, may receive a third enablesignal EN[3], fifth enable signal EN[5], or seventh enable signal EN[7],to reset the first photosensor 752 at the beginning, near the middle,towards the end, or near the end, respectively, of the frame timeT_(F1). The second photosensor reset transistor 765 will receive thesecond enable signal EN[2], and based on the assigned brightness value,may receive a fourth enable signal EN[4], sixth enable signal EN[6], oreighth enable signal EN[8], to reset the second photosensor 753 at thebeginning, near the middle, towards the end, or near the end,respectively, of the frame time T_(F2).

Pixel reset occurs at the end of the frame time T_(F1) for a given row.The reset control signal RESET is pulsed at RESET₁ to the gate of thereset transistor 756 to reset the floating diffusion region N. The firstpixel reset signal V_(rst1) is sampled during RESET₁ by applying thecharge on the floating diffusion region to the gate of the sourcefollower transistor 758 to generate the first pixel reset signalV_(rst1). A row select signal ROW is applied to the row selecttransistor 360 to output the first pixel reset signal V_(rst1). Thefirst pixel reset signal V_(rst1) is then stored by column readoutcircuitry (not shown) connected to a column line, which is connected tothe pixel 750.

After this, the first transfer control signal TX1 and the enable signalEN[1] are pulsed to transfer any charge accumulated from the firstphotosensor 752 to the floating diffusion region N. The charge on thefloating diffusion region N is applied to the gate of the sourcefollower transistor 358 to generate the first pixel image signalV_(sig1). The row select signal ROW is applied to the row selecttransistor 760 to output the first pixel image signal V_(sig1). Thefirst pixel image signal V_(sig1) is stored in column circuits (notshown) connected to the column line, which is connected to pixel 750.The next integration period T_(F1) for pixels in a given row starts whenTX1 is off.

The first pixel reset signal V_(rst1) and the first pixel image signalV_(sig1) are then subtracted to produce a differential signal and thedifferential signal is digitized and sent to an image processor. Thegain of the digital signal is then adjusted by the image processor usinga value that correlates to the brightness value to represent the actuallight intensity to which the first photosensor 752 was exposed.

A similar process is followed to output the second pixel reset signalV_(rst2) during the second reset control signal RESET₂, and to outputthe second pixel image signal V_(sig2) during the second transfercontrol signal TX2 and the second enable pulse EN[2] and further processthe signals.

FIG. 15 shows system 900, a typical processor system modified to includean imager 800 constructed and operated in accordance with an embodiment.The imager 800 has the overall architecture illustrated in FIG. 2, butis modified to include pixel array 230. The processor system 900 is asystem having digital circuits that could include image sensor devices.Without being limiting, such a system could include a computer system,camera system, scanner, machine vision, vehicle navigation, video phone,surveillance system, auto focus system, star tracker system, motiondetection system, image stabilization system, and data compressionsystem.

System 900, for example a digital still or video camera system,generally comprises a central processing unit (CPU) 902, such as amicroprocessor for conducting camera functions, that communicates withone or more input/output (I/O) devices 906 over a bus 904. Imager 400also communicates with the CPU 902 over the bus 904. The processorsystem 900 also includes random access memory (RAM) 910, and can includeremovable memory 915, such as flash memory, which also communicate withthe CPU 902 over the bus 904. The imager 800 may be combined with theCPU processor with or without memory storage on a single integratedcircuit or on a different chip than the CPU processor.

The processes and devices described above illustrate preferred methodsand typical devices of many that could be used and produced. However, itis not intended that the present invention be strictly limited to theabove-described and illustrated embodiments. Any modification, thoughpresently unforeseeable, of the present invention that comes within thespirit and scope of the following claims should be considered part ofthe present invention.

1. An imager, comprising: a pixel for providing an image output signal,said pixel comprising: a photosensor for accumulating charge; a storageregion for storing charge transferred from the photosensor; a transfertransistor for transferring charge from the photosensor to the storageregion, wherein a drain of the transfer transistor is connected to thephotosensor and a source of the transfer transistor is connected to thestorage region; a photosensor reset transistor connected between aselectively enabled control line and a gate of said transfer transistor,the photosensor reset transistor for transmitting a first control signalto a gate of the transfer transistor to reset the charge accumulated onthe photosensor in accordance with said first control signal and asecond control signal applied to a gate of said photosensor resettransistor.
 2. The imager of claim 1, further comprising a controllerfor delivering the first control signal to reset said photosensor duringthe frame time at a timing that corresponds to the value of lightintensity sensed by the photosensor.
 3. The imager of claim 1, furthercomprising: a reset transistor for resetting the charge on the storageregion, wherein a drain of the reset transistor is connected to a supplyvoltage and a source of the reset transistor is connected to the storageregion.
 4. The imager of claim 1, further comprising: a source followertransistor for converting the charge stored in the storage region to avoltage, wherein a gate of the source follower transistor is connectedto the storage region.
 5. The imager of claim 4, further comprising: arow select transistor for outputting the voltage generated by the sourcefollower transistor, wherein a drain of the row select transistor isconnected to a source of the source follower transistor.
 6. The imagerof claim 1, further comprising an array of said pixels.
 7. a processorsystem, comprising: a processor; and an imager coupled to saidprocessor, said imager comprising: a pixel for providing an image outputsignal, said pixel comprising: a photosensor for accumulating charge; astorage region for storing charge transferred from the photosensor; atransfer transistor for transferring charge from the photosensor to thestorage region; and a photosensor reset circuit for selectivelyresetting charge on said photosensor during an image capture period at atime related to a light level received by said photosensor; and aphotosensor reset transistor connected between a selectively enabledcontrol line and a gate of said transfer transistor, the photosensorreset transistor for transmitting a first control signal to a gate ofthe transfer transistor to reset the charge accumulated on thephotosensor in accordance with said first control signal and a secondcontrol signal applied to a gate of said photosensor reset transistor.8. The processor system of claim 7, wherein the processor system is adigital camera.
 9. The processor system of claim 8, wherein the digitalcamera is a still camera.
 10. The processor system of claim 8, whereinthe digital camera is a video camera.
 11. The processor system of claim7, further comprising a controller for controlling the photosensor resetcircuit to reset the charge on said photosensor earlier in the imagecapture period if the light level is lower and later in the imagecapture period if the light level is higher.
 12. The processor system ofclaim 7, wherein the imager further comprises an array of said pixels.